1. Methods of Making Porous Silicon
Porous silicon is a material formed on a surface of bulk silicon by forming multiple nanometer-sized pores using a chemical or electrochemical etching process. A standard electrochemical technique for making such porous silicon is the anodization of silicon. Anodization involves the application of a potential to a bulk silicon sample (e.g., a silicon wafer). For this anodization process, the wafer is immersed in an electrolyte (etching solution) which is commonly a mixture of hydrofluoric acid, water and other components. The anodization process requires a continuous and conducting sample of silicon so that it can be immersed in an electrolyte and a positive potential can be applied (Canham, Appl. Phys. Lett., 57, 1046 (1990)). This standard anodization technique is not, however, capable of producing porous silicon powder due to the inability of establishing electrical contact between particles.
It would be beneficial to have a material with a large specific surface area with respect to the weight of bulk silicon, and which also has a porous layer. An example of such a material is silicon powder with a layer or layers of porous silicon covering the surface of the particles which make up the powder. Such a porous silicon powder would have a much greater porosity-to-weight ratio than an anodized silicon wafer surface. Standard anodization techniques are not, however, capable of making such a porous silicon powder.
2. Methods of Making Silicon Nanoparticles
There are a number of methods currently known for making silicon nanoparticles. These include furnace (Ostraat et al., Appl. Phys. Lett. 79, 433 (2001)) and laser-assisted pyrolysis of silane (Ehbrecht et al., Phys. Pev. B 59, 2975 (1999)), spark processing (Hummel et al., Appl. Phys. Lett. 61, 1965 (1992)), plasma-enhanced chemical vapor deposition (PE CVD) with hydrogen-diluted silane (Tong et al., Appl. Phys. Lett. 69, 596 (1996)), ball milling (Lam et al., J Cryst. Growth 220, 466 (2000)), laser ablation (Fowkles et al., Appl. Phys. Lett. 80, 3799 (2002)), thermal evaporation, RF plasma deposition (Tanenbaum et al., Appl. Phys. Lett. 68, 1705 (1996)), SiO disproportionation (Mamiya et al., J. Cryst. Growth 237–239, 1909 (2002)), and dispersion of a porous silicon layer (Nayfeh et al., Appl. Phys. Lett. 77, 4086 (2000); Credo et al., Appl. Phys. Lett. 74, 1978 (1999)). Most of these methods require either expensive equipment with high maintenance costs and/or they provide relatively low yield, considerably limiting the use of these materials for applications that require bulk quantities.
Among the aforementioned methods, the technique of dispersion of nanoparticles from a porous anodized silicon layer is merely one of the most cost-effective, since it requires simple equipment to produce silicon quantum dots (QDs). However, the product yields by this method are low. It is estimated that approximately one monolayer of nanoparticles is obtained from a porous layer formed on a single wafer surface (Nayfeh et al.; Credo et al.). Thus, the yield (mass efficiency) per run in this method is only about 10−5–10−6 of the mass of the precursor material (silicon wafer) which makes this approach practically unusable for mass production.
A method of creating silicon nanoparticles from a porous silicon powder using the dispersion techniques described above would be deemed advantageous in that larger quantities of the silicon nanoparticles could be generated much more efficiently. This would provide for an increased level of availability and a corresponding increase in their use for research, industrial, and consumer product applications.